This invention relates to drilling fluids and methods for drilling boreholes. More particularly, the invention relates to reduced abrasion thermally stable drilling fluids which include a first additive that absorbs onto the surface of the weighting agent thus reducing abrasion.
The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of strata.
The drilling fluid or mud primarily functions as a medium to carry these fragments of strata or cuttings produced by the drill bit to the surface. Additionally, the drilling fluid serves as a lubricant for the bit and the drill string, and prevents the ingress of formation fluids such as oil, gas and salt water into the borehole while drilling proceeds.
Oil based drilling fluids are used worldwide in on shore and off shore drilling locations. Oil based drilling fluids are utilized because of their capacity for shale inhibition, their high lubricity, their ability to biodegrade in aerobic and anaerobic environments and for their tendency to maintain rheologic properties at high temperatures and high pressures. For these reasons, oil based muds are used to drill deep subterranean wells.
In order to prevent the collapse of the borehole and ingress of formation fluids while drilling deep subterranean wells using oil based muds, hydrostatic pressure in the fluid column must be maintained. The hydrostatic pressure must be maintained throughout the fluid column to the bottom hole. The pressure is maintained by increasing the density of the drilling fluid through the addition of high concentrations of weighting agents. These weighting agents are generally in the form of finely divided solids of a material which has a high intrinsic density.
High concentrations of weighting agents increase the adverse rheologic properties, such as plastic viscosity (Vp), of the drilling fluid. The high concentrations of weighting agents also reduce the amount of cuttings the drilling fluid can transport. The concentration of the weighting agent is related to its specific gravity. If the specific gravity of the weighting agent is low the concentration of the weighting agent in the drilling fluid will be higher.
For example: A drilling fluid mud with a density of 12 ppg (pounds per gallon) is split into two samples. Sample 1 includes a weighting agent of barite with a specific gravity of 4.2. Sample 2 includes a weighting agent of calcium carbonate with a specific gravity of 2.4. Sample 1 with the weighting agent of higher specific gravity and thus the lower concentration of weighting agent in the mud will produce better rheological properties than Sample 2 with the lower specific gravity of weighting agent and thus the higher concentration of weighting agent in the mud.
To create a drilling fluid with excellent rheologic properties, weighting agents of higher density and higher specific gravity are employed. Minerals with high specific gravities are hard and abrasive. These properties lead to erosion of drilling equipment such as mud pumps, directional tools and drill strings.
It is therefore clear that the need remains for a reduced abrasion drilling fluid that is stable at high temperatures and high pressures.